This invention relates to bistable nematic liquid crystal devices.
Liquid crystal devices typically comprise a thin layer of a liquid crystal material contained between cell walls. Optically transparent electrode structures on the walls allow an electric field to be applied across the layer causing a re-ordering of the liquid crystal molecules.
There are three known types of liquid crystal material, nematic, cholesteric, and smectic each having a different molecular ordering. The present invention concerns devices using nematic materials.
In order to provide displays with a large number of addressable elements it is common to make the electrodes as a series of row electrode on one wall and a series of column electrodes on the other cell wall. These form e.g. an x, y matrix of addressable elements or pixels and, for twisted nematic types of devices, are commonly addressed using rms. addressing methods.
Twisted nematic and phase change type of liquid crystal devices are switched to an ON state by application of a suitable voltage, and allowed to switch to an OFF state when the applied voltage falls below a lower voltage level, i.e. these devices are monostable. For a twisted nematic type of device (90xc2x0 or 270xc2x0 degree twist as in U.S. Pat. No. 4,596,446), the number of elements that can be rms. addressed is limited by the steepness of a device transmission vs voltage curve as details by Alt and Pleschko in IEEE Trans ED vol ED 21 1974 pages 146-155. One way of improving the number of pixels is to incorporate thin film transistors adjacent each pixel; such displays are termed active matrix displays. An advantage of nematic type of devices is the relatively low voltage requirements. They are also mechanically stable and have wide temperature operating ranges. This allows construction of small and portable battery powered displays.
Another way of addressing large displays is to use a bistable liquid crystal device. Ferroelectric liquid crystal displays can be made into bistable device with the use of smectic liquid crystal materials and suitable cell wall surface alignment treatment. Such a device is a surface stabilised ferroelectric liquid crystal device (SSFELCDs) as described by:- L J Yu, H Lee, C S Bak and M M Labes, Phys Rev Lett 36, 7, 388 (1976); R B Meyer, Mol Cryst Liq Cryst. 40, 33 (1977); N A Clark and S T Lagerwall, Appl Phys Lett, 36, 11, 899 (1980). One disadvantage of ferroelectric devices is the relatively large voltage needed to switch the material. This high voltage makes small portable, battery powered displays expensive. Also these displays suffer from other problems such as lack of shock resistance, limited temperature range and also electrically induced defects such as needles.
If bistable surface anchoring can be achieved using nematics then a display can be made which has the merits of both the above mentioned technologies but none of the problems.
It has already been shown by Durand et al that a nematic can be switched between two alignment states via the use of chiral ions or flexoelectric coupling: A Charbi, R Barberi, G Durand and P Martinot-Largarde, Patent Application No WO 91/11747, (1991) xe2x80x9cBistable electrochirally controlled liquid crystal optical devicexe2x80x9d, G Durand, R Barberi, M Giocondo, P Martinot-Largarde, Patent Application No WO 92/00546 (1991) xe2x80x9cNematic liquid crystal display with surface bistability controlled by a flexoelectric effectxe2x80x9d. These are summarised as follows:
In Patent Application No WO 91/11747 a device is described with the following characteristics:
1. The cell is made using two surfaces which have SiO coatings of appropriate thickness and evaporation angle to allow two stable states to exist on each surface. Furthermore the two states on a surface are designed to differ in azimuthal angle by 45xc2x0 and the surfaces are oriented to differ in azimuthal angle by 45xc2x0 and the surfaces are oriented such that each of the two resulting domains are untwisted.
2. The cell (of 6 xcexcm thickness) is filled with 5CB doped with 0.5% benzyl quininium bromide and 1.8% phenyl lactic acid. The former is an electrically positive chiral ion with left hand twist while the latter is a negative chiral ion with a right hand twist. The concentrations ensure that the final mixture has a very long pitch so that the states in the thin cell are uniform.
3. Application of a 110V dc pulse for 40 xcexcs enabled switching between the two states. A lower threshold is observed for longer pulse e.g. an 80V threshold is observed for 300 xcexcs pulses.
4. Addition of suitably oriented polarisers caused one state to appear black while the other appears white with a contrast ratio of about 20.
5. A variant device is also -mentioned which causes a short pitch chiral ion mixture between monostable surfaces which possess different zenithal anchoring energies. Switching between a 180xc2x0 twisted state and a uniform state is observed in a 4 xcexcm cell for pulses over 50V.
In Patent Application WO 92/00546 a device is described with the following characteristics:
The cell is made using two surfaces which have SiO coatings of appropriate thickness and evaporation angle to allow two stable states to exist on each surface. Furthermore the two states on a surface are designed to differ in azimuthal angle by 45xc2x0 and the surfaces are oriented such that each of the two resulting domains are untwisted.
The surfaces are also oriented in such a way that the pretilted state on one surface lines up with the untilted state on the other surface and vice versa. Hence when filled with 5CB, the two states are seen as shown in FIGS. 7B and 7C.
Application of a 14V dc pulse across a 1 xcexcm cell for 100 xcexcs allows switching between the states. The final state is dependent on the sign of the pulse due to its coupling to the flexoelectric polarisation. The same voltage threshold is observed for switching in both directions.
The surface used by Durand to obtain bistable alignment was a thin layer of Sio evaporated at a precise oblique angle. However this method suffers the disadvantage that any deviation in the evaporation angle, layer thickness or indeed any of the deposition parameters is likely to produce a surface with only monostable alignment. This makes the oblique evaporation technique unsuitable, or very difficult, for large area displays.
U.S. Pat. No. 4,333,708 describes a multistable liquid crystal device in which cell walls are profiled to provide an array of singular points. Such substrate configurations provide multistable configurations of the director alignments because disclination must be moved to switch between stable configurations. Switching is achieved by application of electric fields.
Another bistable nematic device is described in GB.2,286,467-A. This uses accurately formed bigratings on at least one cell wall. The bigrating permits liquid crystal molecules to adopt two different angular aligned directions when suitable electrical signals are applied to cell electrodes, e.g. dc coupling to flexoelectric polarisation as described in Patent Application No. WO.92/00546. Since in the two splayed state the director is quite close to being in the plane of the layer, the coupling between director and flexoelectric component can be small, which may hinder switching in some circumstances.
According to this invention the above disadvantages are overcome by a surface treatment to at least one cell wall that permits nematic liquid crystal molecules to adopt either of two pretilt angles in the same azimuthal plane. The cell can be electrically switched between these two states to allow information display which can persist after the removal of power.
The term same azimuthal plane is explained as follows; let the walls of a cell lie in the x,y plane, which means the normal to the cell walls is the z axis. Two pretilt angles in the same azimuthal plane means two different molecular positions in the same x,z plane.
According to this invention a bistable nematic liquid crystal device comprises;
two cell walls enclosing a layer of liquid crystal material;
electrode structures on both walls;
a surface alignment on the facing surfaces of both cell walls providing alignment to liquid crystal molecules;
means for distinguishing between switched states of the liquid crystal material;
CHARACTERISED BY
a surface alignment grating on at least one cell wall that permits the liquid crystal molecules to adopt two different pretilt angles in the same azimuthal plane.;
the arrangement being such that two stable liquid crystal molecular configurations can exist after suitable electrical signals have been applied to the electrodes.
The grating may have a symmetric or an asymmetric groove profile.
The grating may have an asymmetric groove profile which will induce a pretilt of less than 90xc2x0, e.g. 50xc2x0 to 90xc2x0. An asymmetric profile may be defined as a surface for which there does not exist a value of h such that;
xcexa8x(hxe2x88x92x)=xcexa8x(h+x)xe2x80x83xe2x80x83(1)
for all values of x, where xcexa8 is the function describing the surface.
The gratings may be applied to both cell walls and may be the same or different shape on each wall. Furthermore the grating profile may vary within each pixel area, and or in the inter pixel gaps between electrodes. One or both cell walls may be coated with a surfactant such as lethecin.
The liquid crystal material may be non twisted in one or both stable molecular configurations.
The cell walls may be formed of a relatively thick non flexible material such as a glass, or one or both cells walls may be formed of a flexible material such as a thin layer of glass or a plastic material flexible e.g. polyolefin or polypropylene. A plastic cell wall may be embossed on its inner surface to provide a grating. Additionally, the embossing may provide small pillars (e.g. of 1-3 xcexcm height and 5-50 xcexcm or more width) for assisting in correct spacing apart of the cell walls and also for a barrier to liquid crystal material flow when the cell is flexed. Alternatively the pillars may be formed by the material of the alignment layers.
The grating may be a profiled layer of a photopolymer formed by a photolithographic process e.g M C Hutley, Diffraction Gratings (Academic Press, London 1982) p 95-125; and F Horn, Physics World, 33 (March 1993). Alternatively, the bigrating may be formed by embossing; M T Gale, J Kane and K Knop, J App. Photo Eng, 4, 2, 41 (1978), or ruling; E G Loewen and R S Wiley, Proc SPIE, 88 (1987), or by transfer from a carrier layer.
The electrodes may be formed as a series of row and column electrodes arranged and an x,y matrix of addressable elements or display pixels. Typically the electrodes are 200 xcexcm wide spaced 20 xcexcm apart.
Alternatively, the electrodes may be arranged in other display formats e.g. r-xcex8 matrix or 7 or 8 bar displays.